This invention relates to a system for the control of electrical apparatus with particular reference to control by electric sensors, and with special reference to a control system for domestic appliances.
Over the past 20 years, household appliances have not changed greatly in the way that they are controlled. They have always used motor timers with cam contacts, and bimetal, gas tube, or captive expansible liquid temperature sensors. In recent years, electronics have played a greater and greater role in improving the control capabilities of consumer products. More recently, the cost of microelectronics has fallen to a level where they can now be introduced to the massive appliance market. Microelectronics give the manufacturer the ability to enhance the performance of a product and at the same time, provide features that were previously cost prohibitive.
Here described is a controller system and associated novel equipment, initially designed for combination in the domestic refrigerator. It should be noted that with relatively minor changes it may be adapted to control other appliances such as a dishwasher, dryer, washer, stove, etc. The preferred embodiment is microcomputer based.
The standard domestic refrigerator is now well established and consists basically of two compartments, though there may be other compartments for specialized use, such as for custom ice making, ice cream freezing, etc.
An electric unit produces the cold, typically a motor driven compressor with external condenser unit and internal evaporator, with a rather crude method of controlling the compressor. The most common temperature control has been an electromechanical device having a bellows type cylinder, and a capillary tube filled with a liquid which expands on heating operating the bellows device to turn the compressor motor on and off. Periodically, frost which builds upon the evaporator from humidity in the environmental air must be removed to maintain cooling efficiency.
The first basic compartment is a freezer section to be maintained at approximately 0.degree. F. (-18.degree. C.) for the storage of frozen foods, and in which ice is made automatically or by the placing of trays of water to be frozen when required. The second basic compartment, in which the thermostat is placed, is a food section maintained slightly above freezing in the range of 33.degree. to 39.degree. F. (0.5.degree. to 4.degree. C.) and in which food is stored at a temperature which keeps bacterial growth and mould spoilage to a minimum, without producing any change of state of the food which would occur on freezing. The lower sections of the compartment are cooler than the upper sections because of the convective air flow. Certain sections of the food compartment are at various temperatures in this range, achieved by providing lids, by open or closed shelves, or by being formed within the wall or door and, therefore, subject to heat infiltration from outside. Yet other sections, such as a butter keeper, may be supplied with supplemental heat, for instance by an electric heater, so that butter can be maintained in a condition for easy spreading.
The preferred popular refrigerator is of the so-called "frost-free" type in which the user does not have to carry out the chore of defrosting and removing melted frost from the evaporator. In these systems, air is circulated over a finned evaporator unit placed adjacent to or in the freezer compartment, so that the cold air so formed can be delivered by a circulating fan to the freezer, and constitutes essentially the only heat exchange medium for the goods in the freezer. A fraction of the cooled air from the evaporator is routed to provide cooling for the food compartment. The circulating fan runs when the compressor is running. Frost build-up on the evaporator is removed at regular intervals by a heater, controlled from a timer, which provides for regular defrost every four to twenty hours depending upon the particular model concerned. The defrost cycle usually allows a period of about 20 minutes during which the evaporator is heated to a limit temperature of about 70.degree. F. (20 C.). The wattage needed to effect the defrost is usually fairly high so that the ice is removed in a short time period and the cooled compartments do not warm unduly. The melted water is routed to a pan near the condensor so that the water will evaporate to the environment over the next few hour period.
A fault with the frost-free system is that each day, regardless of whether or how often the door has been opened, the refrigerator goes through its defrost cycle. The owner may be away on holidays for a period of one month--nevertheless the refrigerator defrosts every day or several times every day. Another drawback of this frequent defrosting is that the circulating air becomes very dry and any food in the refrigerator has a tendency to lose its moisture and the condition known as "frost burn" occurs.
A method of defrosting, which has some advantages to the timed daily cycling, provides that, on opening the door, a switch button is actuated and a pulse initiated. The pulses are recorded and after a predetermined number, the unit is caused to defrost. The disadvantage is that on humid days, the unit can frost up far more quickly than expected, so that operation become inefficient and in the last resort can be less satisfactory than the previously described timed method.
Only the food compartment is under thermostatic control, yet the main body of the cooling air is circulated through the freezing compartment, and the temperature of the freezer is dependent upon the running time of the compressor caused by the amount of use, door opening, thermal load, etc. of the food compartment. There is a hand operated damper for determining the fraction of cold air from the evaporator which is passed into the food compartment, but, even so, there are wide variations in temperature experienced in the freezer compartment. Furthermore, even if the freezer door is left open for periods which bring it above freezing, there is no sensing of the rise in temperature seen by the food compartment thermostat. The freezer temperature is thus under very haphazard control at best.
Refrigerators also have what is called a mullion heater, located in the mullion around the freezer compartment door and sometimes also around the food compartment door. The purpose of the heater is to remove excessive cold from the surface adjacent the door opening so that when struck by warm moist environmental air, moisture does not condense out around the door. Most units have a manual switch which the owner is instructed to turn on when required. The switch is in practice either on continuously, or not on at all, as most householders do not appreciate its significance. One method of controlling the mullion heater has been to turn it on whenever the compressor unit is on. In this there is no correlation to the presence of moisture.
A problem also arises with the butter compartment. Normally there is a three-position switch (OFF/LO/HI) which can supply appropriate heat so that butter is relatively soft. However, the user has a choice of only three positions, people's preferences are different, and the temperature depends on the quantity of butter to be tempered. The arrangement is thus less than satisfactory.
In general, it is, therefore, true to say that the current domestic refrigerator suffers from several drawbacks. The temperature it provides in its freezer compartment can vary widely, and it may be required to defrost excessively often in dry weather if undue frost build-up in humid conditions is to be avoided. Excessive defrost activity is wasteful in requiring extra refrigerator operation and also gives more frequent rises in temperature in the freezer compartments than are desireable. It is wasteful in that heaters for the mullion and the butter compartment are either permanently on or switched off so that their advantages are not achieved. It may also be called upon to run more often or longer than necessary if the food compartment requires more than average cooling while the freezer does not.
Servicing of the refrigerator often involves more time and experimentation by the service man than would be necessary if some accurate idea of its failure could be given. Non-technical owners are notoriously inaccurate in their assessment of actual faults occuring in their domestic appliances.